This invention relates to a method for nondestructively characterizing the uniformity of imperfection levels in high resistivity semiconductor wafers which are for use in the fabrication of integrated circuits or optoelectronic devices.
Profiling techniques capable of detecting dominant imperfection distributions or levels in semiconductor materials are particularly important because these imperfections often influence device performance. Dominant imperfection distributions or levels for high resistivity semiconductors are characterized as deep donors or acceptors which compensate for free electrons or free holes in the semiconductor wafers resulting in charge neutrality. Most of the currently used techniques do not produce data suitable for directly comparing wafer quality and device performance because of their destructive nature or requirements for low temperatures during data measurements.
Commonly used profiling techniques include Hall effect measurements, Deep Level Transient Spectroscopy (DLTS), Photoluminescence (PL), and Fourier Transform Infrared Absorption (FTIR). Drawbacks of using Hall effect measurements and DLTS methods for profiling lie in the fact that they are not only destructive but also require time consuming sample preparation and data acquisition. While PL can be used for nondestructive profiling, measurements must be carried out at very low temperatures. In addition, only imperfections with luminescence behavior can be detected. FTIR has also been used for detecting various imperfections; however, FTIR requires a thicker layer of wafer material in order to have significant infared absorption. Therefore, device processing is not possible immediately following FTIR measurements.
An electrical profiling technique described by R. T. Blunt, S. Clark, and D. J. Stirland in an article entitled "Dislocation Density and Sheet Resistance Variations Across Semi-Insulating GaAs Wafers" published in the IEEE Transactions on Electron Devices, Vol. ED-29, No. 7, July 1982 has been used to measure sheet resistances across wafers cut from Liquid Encapsulated Czochralski (LEC) pulled semi-insulating GaAs boules. The technique involves the use of a "dark spot" for electrical uniformity mapping. A uniformly illuminated strip of GaAs having a contact at each end is fitted with a moveable mask; the moveable mask prevents the light from reaching a small portion of the GaAs (i.e., creating a dark spot on the GaAs). The resistance is measured across the dark spot under the moveable mask. An electrical profile of the GaAs strip is obtained by moving the mask along the illuminated strip. The method is extended to cover complete wafers by using a fixed mask to prevent light from reaching the material on either side of the uniform width strip to be measured. The concept is also extended to cover sheet resistance mapping over the complete area of a circular wafer by sequentially illuminating and scanning strips between opposite pairs of contacts.